1. Field of the Invention
The present invention relates to a battery power supply system, and, more particularly, to a buffered battery charger circuit capable of controlling the power supplied to an active system and to a rechargeable battery. Particular utility of the present invention is in a power supply system for portable electronic units; although other utilities are contemplated herein.
2. Description of Related Art
FIG. 1 is a simplified block schematic of a typical prior art power supply topology 20 for a portable electronic system 24. The active system 24 gets power, conditioned by the system DC/DC converter 22, either from the battery 18 or from the external input power adapter 10. The input power adapter 10 gets the power from an external primary power source, such as an AC outlet or a DC source, and provides the power directly to both the system DC/DC converter 22, through the separating diode 12, and to the battery charger 14. The battery 18 is connected and provides power to the system DC/DC converter 22 through the separating diode 16, as long as the primary power source is not available. When the primary power source is available, the battery is isolated from the power input of the system DC/DC converter 22 by the reversed polarized (reversed biased) diode 16. In addition, the battery 18 is charged when power is supplied by the primary power source, through the charger 14. This topology in FIG. 1 has the disadvantage of big and fast voltage transients at the node 25, which is the input of the system DC/DC converter 22.
FIG. 2 shows a simplified block diagram of a buffer battery power supply 20' topology. The battery pack 18 is permanently connected to the input of the system DC/DC converter 22 and provides the requested power. The external input power adapter 10 powers the battery charger 14 when an external primary power source is available. The external input power adapter 10 is intended to adapt the parameters of the primary source to the charger input requirements. The battery charger 14 powers in parallel both the system DC/DC converter 22 and the battery 18 to charge it or to maintain the voltage of the fully charged battery at the optimal level. This "buffer battery topology" limits the voltage variations at the system DC/DC converter input (node 25) to normal battery pack voltage variations and does not allow fast voltage transients at this input. Furthermore, when the power requested by the system 24 temporarily exceeds the capability of the input power adapter 10, both the input power adapter 10 and the battery 18 will deliver in parallel the power to the system 24 through the converter 22. Disadvantageously, however, the circuit 20' shown in FIG. 2 provides no mechanism by which the power supplied by the battery charger can be reduced or increase based on preset limits or demand from the battery, the system, or both.
Similarly, U.S Pat. No. 5,698,964 issued to Kates et al. provides a battery charging circuit topology. This circuit monitors the current from an AC adapter (i.e., I.sub.in) and adaptively utilizes all available current to charge the batteries. The system DC/DC converter is powered directly by the AC adapter after its connection; the battery is disconnected from the system. Thus the voltage at the input of the system DC/DC converter abides a heavy transient, from the low voltage of a discharged battery to the AC adapter voltage, every time higher than the maximum charged battery voltage. Furthermore, as the AC adapter output voltage could vary, no real control is provided for the power delivered by the AC adapter to both the system (e.g. portable electronic device) and the battery. A similar topology is provided in U.S. Pat. No. 5,723,970 issued to Bell, which suffers similar and for additional drawbacks mentioned above.
Thus, there exists a need to provide a buffered battery power supply system that can control both the total output power and the power delivered to the battery. Moreover, there exists a need to provide a system that will significantly reduce the voltage transients that may appear at the electronic device, the battery, or both.